Transplantation has emerged as the preferred method of treatment for many forms of end-stage organ failure. While the short-term results have improved long-term outcomes remain inadequate. To maintain their allografts, patients must rigidly adhere to life-long treatment regimens using costly immunosuppressive agents that dramatically increase the risks of cardiovascular disease, infections and malignancies. The development of strategies to promote the acceptance of allogeneic tissues without the need for chromic immunosupression could not only reduce the risk of these lifethreatening complications, but also greatly expand the application of organ, tissue and cellular transplantation for diseases such as the hemoglobinopathies and genetic immunodeficiencies, Type I diabetes, and possibly other autoimmune diseases. We have developed novel nonmyelosuppressive protocols using axiti-CD40L and CTLA4-Ig to permit the induction of titratable levels hematopoietic chimerism and robust deletional donor-specific tolerance in rodents. The goal of this project is to develop and optimize protocols to induce stable macrochimerism and transplantation tolerance to renal allografts in non-human primates. Specifically, in this proposal we will test the effects recipient conditioning with non or minimally myelosuppressive doses of busulfan or total body irradiation on the level and durability of hematopoietic chimerism when used in conjunction with a regimen consisting of anti-CD40L, CTLA-Ig, sirolimus and donor bone marrow, 2) determine whether escalating doses of G-CSF mobilized CD34+ cells will a) increase the level of hematopoietic chimerism, b) decrease the requirement for recipient conditioning, and c) induce transplantation tolerance to renal allografts in MHC-disparate Rhesus macaques and 3) determine the effects of an optimized tolerance induction protocol on the survival of concurrently placed renal allografts, anti-donor immune responses and protective memory T cell responses in MHC-disparate Rhesus macaques.